Feb 19, 2019

Whittier Narrows Dam No Longer Meets Tolerable-Risk Guidelines

The U.S. Army Corps of Engineers has prioritized strengthening the California flood control structure

USACE study finds flood control structure near Los Angeles may fail in 900-year storm
USACE study finds flood control structure near Los Angeles may fail in 900-year storm

In a series of public hearings, the U.S. Army Corps of Engineers (USACE) revealed that the 60-year-old Whittier Narrows Dam in Pico Rivera, Calif., near Los Angeles, with a population of approximately 63,000, no longer meets the agency’s tolerable-risk guidelines and could potentially fail in the event of a 900-year storm.

USACE found that the dam could fail if water flows over its crest or if seepage erodes sandy soil below. Additionally, a large storm could lead to a premature opening of the dam’s spillway on the San Gabriel River, which is among the steepest rivers in the United States, potentially releasing more than 20 times what the downstream channel can contain, reported The Los Angeles Times.

Expanding on that, the Los Angeles times reported that for heavily populated areas of the Los Angeles Basin, increased runoff from the San Gabriel Mountains could overwhelm a flood control dam on the San Gabriel river, releasing floodwaters from Pico Rivera to Longbeach.

 In response, the federal agency is seeking up to $600 million in federal funding to upgrade the 3-mile-long dam. The project has been classified as a high priority, due to the dam failure’s potential to completely inundate parts of the Los Angeles Basin from Pico Rivera to Long Beach. 

Funding will require congressional approval, Doug Chitwood, the lead engineer on the project said to The Los Angeles Times

A government study used computer models to estimate the impact of 900-year, 7,500 years and 18,000 year storm events on the dam and found the effects could be catastrophic. The U.S. Geological Survey is reporting that a mega-storm would last for weeks and cause more than 1.5 million people to have to flee as floodwaters inundate cities.The U.S. Department of Interior and the U.S. Geological Survey have been working to raise awareness of the threat of mega-storms and encourage emergency planning.

Some scientists believe climate change has increased the frequency of such mega-storms. A recent study by UC Irvine researchers examined 13 California reservoirs and found the risk of dam failure likely to increase in a warming climate, according to The Los Angeles Times. The researchers pointed to a 2017 failure of the Oroville Dam, where 180,000 people were forced to evacuate due to a dam failure.

Now, officials are working with the federal government to develop emergency plans that can be executed before the repair project at the dam is completed in 2026, according to The Los Angeles Times. 

For example, Pico Rivera spokesman Robert Alaniz, told the Los Angeles Times the city would be using a $300,000 grant from the California Department of Water Resources to revise its current evacuation plans that use thoroughfares crossing the San Gabriel River to the east and Rio Hondo to the west. 

Read more about flood control and dam projects.